PHYSICS 50: Lecture 11.1 RICHARD CRAIG

Homework #10 Chapter 10 Chapter 11 Exercises and Problems We will skip 10.7 Chapter 11 Sections 1,2,3 only Exercises and Problems 10.1, 10.16, 10.41, 11.12, 11.14, 11.56 Due Tuesday April 14

Plan for the Day Some thoughts on Quiz of Tuesday Center of Mass and Center of Gravity Equilibrium Static Problems (things don’t move) Simple: Board on benches Medium: Hammer Hard: Hanging sign No Quiz (yeah!) but have short exit test

Center of Mass Formal Definition: If density constant center of mass is just the geometric center of a body If gravity constant center of gravity and center of mass the same point For torque calculations a body acts as if all the weight is at the center of gravity (linear function unlike moment on inertia)

Conditions for Equliubrium Sum of forces must be zero Sum of torques must be zero

Simple Example: Bench What are the forces from each support on the bench?

Idea problem Q11.2 A. less than 0.25 m B. 0.25 m A rock is attached to the left end of a uniform meter stick that has the same mass as the rock. How far from the left end of the stick should the triangular object be placed so that the combination of meter stick and rock is in balance? A. less than 0.25 m B. 0.25 m C. between 0.25 m and 0.50 m D. 0.50 m E. more than 0.50 m

Medium Example: Hammer If I pull the hammer with 50lbs, how much force is on the nail?

Hard Example: Hanging Weight Bar weighs 150N, Wire is massless, What is the tension in the wire

C. T = w/(sin q) D. T = w/(cos q) E. none of the above Tell me the answer to leave today……… A metal advertising sign (weight w) is suspended from the end of a massless rod of length L. The rod is supported at one end by a hinge at point P and at the other end by a cable at an angle q from the horizontal. What is the tension in the cable? A. T = w sin q B. T = w cos q C. T = w/(sin q) D. T = w/(cos q) E. none of the above